During in situ leaching of uranium and other minerals, as the deposit approaches exhaustion, the concentration of uranium (or other elements of value) in the produced solution will decline to a level where the operation is no longer economically feasible. In most such operations it is advantageous to plan and operate the leaching operation in a manner such that the economic cutoff assay of the produced solution is not reached until a high percentage of the mineral or element of interest has been recovered.
A typical in situ leaching operation might consist of an array of 5-spot patterns, each pattern comprising a central production well and 4 corner injection wells. In a contiguous array of a multiplicity of such patterns the corner injection wells are usually common to all of the immediately adjacent patterns. Although diminished uranium (or other mineral) assay of the present solution is to be expected as the well patterns approach exhaustion, low assays also may result from dilution that occurs when part of the injected leachant flows directly to the production well via short and/or high-permeability paths that soon become barren of uranium. At the production well, the dilute solution mixes with uranium-bearing solution that has flowed through more circuitous and/or less permeable paths that do contain leachable uranium. There are at least two modes by which such preferential flow of leachant may occur: (1) the lateral permeability of various horizontal layers within the ore zone may be different, resulting in preferential flow of fluid through those layers that are most permeable, and (2) within any given layer having relatively uniform lateral (typically horizontal) permeability in all directions, flow will tend to be concentrated along the shortest path between the injection wells and the production well because this path has the shortest length and the highest pressure gradient. Mode 2 is shown graphically in FIG. 1.
When preferential leaching entirely by the first mode occurs, there seems to be little that can be done about it presently. Although chemical injection to block off the more premeable layers can be considered, it entails the risk of blocking off ore zones and might considerably complicate efforts to purge objectionable solutions from the mineral zone after mining is terminated. If mode 1 leaching is not predominant, and the variations in lateral permeability over the vertical dimensions are not substantial, the pattern of flow suggests that there are certain areas which contain most of the remaining mineral. For example, FIG. 2 is a theoretical flow network of one quarter of an enclosed 5-spot pattern having uniform lateral permeability. By scaling from FIG. 2, a theoretical estimate has been made of pattern area represented by paths A, B, C, D, and the precentage of the total fluid that flows through each path.
______________________________________ A B C D ______________________________________ % of Pattern Area 21.4 24.5 29.8 24.3 % of Fluid Flow 30 30.6 25 14.4 ______________________________________
Thus, about 25 percent of the uranium is present in those parts of the pattern area through which less than 15 percent of the leachant flows. Using these figures it is estimated that when paths A and B are exhausted some 22 percent of the original uranium will remain in areas represented by paths C and D, and the U.sub.3 O.sub.8 concentration of the solution reaching the production well will be about 40 percent of what is was during the early stages of leaching. When paths A, B, and C are exhausted, U.sub.3 O.sub.8 recovery will be about 93 percent but solution U.sub.3 O.sub.8 assay will have diminished to less than 15 percent of its earlier value. As uranium recovery will be limited to whatever can be extracted before the solution concentration becomes too low for economic processing, it would be desirable to operate the patterns in a manner that will tend to keep solution concentration at a higher level. There is such a mode of operation.
The following U.S. patents have been found in a search performed on this subject:
______________________________________ 3,863,987 2,919,909 2,952,449 3,654,866 3,718,366 3,841,705 3,779,601 3,713,698 3,709,295 3,647,261 3,606,465 3,442,553 3,309,141 2,954,218 3,309,140 2,818,240 ______________________________________
It is believed that two of the references may be of special interest to the reader. These are Bays U.S. Pat. No. 2,952,449, and Livingston, U.S. Pat. No. 2,818,240. The Bays patent discloses a method for forming an underground communication between bore holes; however, the method involves the application of a hydraulic pressure to achieve a fracture of the formation. The removal of fracturing pressure from one hole and placing it on another hole in order to aid in the fracturing of a rock formation is not the same process or approach used by Applicant in his leaching process.
The Livingston U.S. Pat. No. 2,818,240, which is concerned with leaching, describes several different stages of leaching including a "flooding" stage and a "pressure leach" stage. Livingston uses a row by row approach, or the conversion of injection wells to production wells and/or vice versa; he does not close all of one type of well and convert only some of the others as does Applicant in the present case.